This invention concerns catalytic cracking of hydrocarbons. In particular, the present invention concerns an improved process for catalytically cracking hydrocarbon feeds which contain catalyst-deactivating components such as asphaltenic coke precursors and nitrogen compounds.
In present commercial processes for catalytically cracking hydrocarbons in the absence of externally supplied hydrogen, the cracking catalyst is employed in the form of fine particles. The fine catalyst particles are continuously cycled between a cracking reaction zone and a catalyst regeneration zone. In the reaction zone, a stream of hydrocarbon feed is continuously contacted with fluidized catalyst particles, usually at a temperature of about 425.degree. C. to 600.degree. C. Reactions of hydrocarbons at the high temperatures employed cause deposition of coke on the catalyst particles. The resulting cracked hydrocarbons are thereafter separated from the spent, coke-containing catalyst, recovered and further processed by fractionation. The spent catalyst is stripped of volatiles and transferred to the catalyst regeneration zone, there catalytic activity is restored to the catalyst by burning off the coke.
The extent of hydrocarbon conversion obtained in a fluid catalytic cracking operation may be defined as the volume percent of fresh hydrocarbon feed normally boiling above the gasoline endpoint which is changed to products boiling below the gasoline endpoint. The end boiling point of gasoline for the purpose of determining conversion is conventionally defined as 221.degree. C. The extent of conversion is often employed as a measure of the activity of a catalyst used in a commercial FCC operation. At a given set of operating conditions, a more active cracking catalyst provides greater conversion than does a relatively less active catalyst. The ability of a catalyst or a cracking reactor design to provide a greater extent of conversion for a given feedstock is desirable in that it allows an FCC system to be operated in a more flexible manner. For example, with increased conversion, feed rate to a cracking unit may be increased. The present invention provides a method for increasing feed conversion when processing feedstocks containing catalyst-deactivating components.
For the purposes of the present invention, "riser cracking" may be defined as catalytic cracking of hydrocarbons in which cracking takes place while the catalyst and hydrocarbon feed are moving cocurrently upwardly through a relatively small diameter, generally vertically elongated reaction zone, with catalyst particles being transferred through the reaction zone by entrainment in a vapor stream.
Nitrogen compounds are often present in FCC hydrocarbon feeds. Nitrogen-containing compounds are strongly adsorbed on the acidic sites in cracking catalyst during the cracking step, resulting in greatly decreased cracking activity and conversion when processing feedstocks with any appreciable nitrogen content. It has been found that a large proportion of the nitrogen compounds present in an FCC feed are deposited on the catalyst during the cracking step in a form which is not removed from the catalyst particles by steam stripping. This suggests that nitrogen compounds are preferentially strongly adsorbed on catalyst acidic sites during initial contact between the catalyst and feed.
For the purposes of the present invention, "asphaltenes" are defined as the component of a catalytic cracking feedstock which is insoluble in normal heptane and which is nondistillable (i.e., decomposes upon heating to form coke). Asphaltenes are notably present in residual fractions of petroleum distillation. Feedstocks containing asphaltenes are known to deposit coke heavily on the cracking catalyst during processing. This results in rapid loss of cracking activity.
U.S. Pat. No. 4,234,411 discloses a fluid catalytic cracking process employing a split flow of recycled, regenerated catalyst to a riser-reactor. A first portion of the catalyst is introduced into the lower part of the riser and a second portion of the catalyst is introduced into an upper portion of the riser. The relative amounts of catalyst introduced to the different levels of the riser are regulated in accordance with process operating temperatures in, respectively, a downstream reactor separator and in the riser-reactor.
U.S. Pat. No. 4,090,948 discloses a fluid catalytic cracking process using a riser-reactor in which the hydrocarbon feed is first contacted with spent catalyst in an upstream area of the riser-reactor. Regenerated catalyst is then mixed with the feed in a downstream part of the riser-reactor. The patent states that the spent catalyst has sufficient activity so that the highly reactive nitrogen and carbon residue containing hydrocarbon contaminants in the oil feed will deposit on the spent catalyst and thus minimize the deactivation of the active, regenerated catalyst used in the downstream part of the reactor for cracking of the oil feed.